The food freeze dryer freezes the water in the material into a solid by cooling it down, and then sublimates the solid water ice into water vapor directly by reducing the vacuum and gently heating it to achieve the purpose of dehydration. Compared with the dehydration method of heating and drying, freeze drying has many advantages, such as low volatility loss, no need to add preservatives to preserve freshness, maintaining the structure of the material, maintaining the active ingredients in the material, and maintaining the easily oxidized components in the material.
There are three common heating technologies for food freeze dryers:
1. Conduction heating
Conduction heating is mainly based on contact heat transfer. The heat source is electric heating, steam heating, etc., which uses a heat carrier to pass through a heating plate. The direction of heat transfer and mass transfer is the same. As the sublimation surface continues to shrink inward, the dried layer becomes thicker and thicker, and the frozen layer becomes thinner and thinner. Therefore, the resistance to mass transfer becomes greater and greater, while the resistance to heat transfer becomes smaller and smaller. The contact heat transfer efficiency is high, but it is easy to cause excessive heating of the material and affect the product quality. If properly controlled, its defects can be avoided.
2. Radiant heating
Radiant heating is the heat transferred from the heat radiation plate to the surface of the material drying layer through radiation, and then from the surface of the drying layer to the sublimation interface through heat conduction and gas convection. The directions of radiant heat transfer and mass transfer are opposite, and the temperature of the internal frozen layer is determined by the balance of heat transfer and mass transfer. The general characteristic of radiant heating is that as the sublimation surface shrinks inward during the drying process, the thickness of the dried layer becomes thicker and thicker, and the resistance to heat transfer and mass transfer increases at the same time. Radiative heat exchange can overcome the shortcomings of contact heat transfer and is easy to ensure product quality, but it takes a long time and has low efficiency.
3. Microwave heating
Microwave heating is suitable for the freeze-drying process of heat-sensitive materials. Since its heat transfer is basically not affected by the drying layer, the heat energy can directly reach the sublimation surface of the material. Therefore, the temperature difference between the surface of the material and the sublimation surface is very small, which can maintain and approach the high temperature allowed by the material. Therefore, it is beneficial to the drying process, which can greatly shorten the drying time and increase the drying rate. For thick materials, microwave heating is suitable for freeze drying, while for thin materials, surface heating, that is, contact heat transfer and radiation heat exchange, is suitable; as for the dividing point of thickness, it depends on the properties of different materials; microwave drying has the characteristics of uniform heating and short drying time, and has great development potential. However, under the drying chamber pressure of 13-66Pa, microwave heating is prone to cause glow discharge. This phenomenon is likely to cause a “chain reaction” of freeze-dried food, which ultimately leads to the failure of freeze drying, and microwave heating is prone to produce odor in freeze-dried products. Microwave heating is expensive and the process is difficult to control. It is currently in the development stage and is less used in industrial mass drying.
Which heating method is used in the actual freeze-drying process of the food freeze dryer? It is necessary to conduct a specific analysis of various working conditions.